I love them. Once you know how to utilise them, they extremely handy to have. The first thing I taught myself back in 40d after I figured out how to do my food and booze industries was how to breach and use aquifers. Why? Well it was because of all the fear and general hatred of the things I saw on the forums, so I decided it was gonna be my first challenge. My very first aquifer pierce was using the pump method on a 2z-level aquifer on an isolated island. I'd later go on to learn how to cave-in and freeze my way through them as well, though the cave-in is my preferred method now. Well anyways a dozen or so flooded forts later, I think I'm fairly proficient with them.
You can do things like a very quick underground treefarm, farmzone, and even really easy drowning traps.
And some other things given a flat embark, a soil layer aquifer is always 1 z-lvl + its drainage level except in sand deserts where it can potentially be 2 z-lvls + its drainage. If you have 2 soil layers marked as aquifer, it'll be those those 2 z-lvls of aquifer + the drainage below the 2nd one. The aquifer will never appear in the top 2 soil layers. Sandstone, and conglomerate are the only stone layer aquifers with puddingstone the only mineral aquifer. Due to the nature of rock layers which sandstone and conglomerate are, they can be some 3 to 7 z lvls thick meaning 3 to 7 z-lvls of aquifer + the drainage lvl at the bottom. Conglomerate is just a pain, but sandstone is actually fairly easy to use the dig through mineral method on in 31x. Since large clusters appear everywhere right now, and all the large clusters that can appear in sandstone are non-aquiferous, which means if you line your shaft up with the center of a 48x48 embark tile you'll likely be able to dig straight through the bauxite, magnetite, kaolinite, etc clusters of sandstone and be totally dry. Conglomerate doesn't work that way though because of the puddingstone which appears in it with quite a bit of frequency.